1. Field of the invention
The present invention concerns a method of fabricating integrated optical circuits including waveguides having different structures, in particular different vertical and lateral structures. The method improves the coupling between the guides and consequently minimizes optical losses.
2. Description of the prior art
Integrated optical circuits are designed to combine a plurality of optical components having different functions and structures. Such circuits are made in two stages. In a first stage the various active and/or passive components are integrated on a substrate using a method known as butt coupling. In a second stage the waveguides of the various components are etched in the form of strips to assure lateral confinement of the light.
The butt coupling method of integrating optical components is very widely used at present. The schematics of FIGS. 1A through 1D represent views in longitudinal section of an integrated optical circuit during various steps of the butt coupling method.
This method consists, in a first step, of growing onto a substrate 1 various layers 2, 3, 4 intended to form a passive structure, for example. The stack of layers comprises a first layer 2 of indium phosphide (InP), a second layer 3 of quaternary material to serve as a waveguide and finally a third layer 4 of InP, for example.
At least one cavity 5 is then etched into this stack of layers using a conventional and optimized process. The cavity 5 is reserved for the integration of another type of optical component, such as a semiconductor laser, for example.
The active component is formed by growing a plurality of layers 6, 7 and 8 epitaxially. The layer 7 constitutes the waveguide and is formed of a quaternary material, for example, whereas the other layers 6 and 8 are formed of InP, for example. The structure of the waveguide 7 is different from that of the passive guide 3. The interface 9 between the two types of component is known as a butt joint.
At this stage in the fabrication of integrated optical circuits the various optical guides must be etched in the form of strips to assure lateral confinement of the light. Over the last five years many attempts have been made to etch the waveguides using a so-called self-aligning method.
This method consists in defining the lateral structures of the guides in a single step using a single mask 10 as shown by the schematic top view of a circuit shown in FIG. 2. The region ZA corresponds to the active area and the region ZP corresponds to the passive area.
However, the self-aligning method requires a compromise with regard to the process for producing the waveguide strips. This compromise is not without consequences in terms of the performance of the final device.
In the conventional process the various active and passive optical components are fabricated separately and the structures of the waveguides are defined by completely different and optimized technologies. When two different structures are placed end to end and their waveguides are etched by means of a single mask 10 only one technology is generally used, rather than two.
The composition of the mask 10 used in the self-aligning process is generally different from that of the masks used in conventional processes for fabricating each of the optical components separately. Its composition can preferably be metallic rather than of resin or of silica. This difference in composition leads to a modification in the etching technology employed to define the structures of the waveguides. Accordingly, the self-aligning method requires a compromise with regard to the etching technology for defining all the waveguide structures simultaneously.
The attempts made until now to obtain a satisfactory compromise have not been successful and in no single instance indicate that mass production of integrated optical circuits is feasible.
A first problem to be solved therefore consists in finding another solution for producing integrated optical circuits using the fabrication technologies routinely employed and optimized for each of the components constituting such circuits.
We have envisaged integrating the various optical components by the butt coupling method and then defining the lateral structures of the waveguides by a realignment method. The realignment method more precisely consists in defining the waveguides having different structures using a plurality of masks, possibly of different composition, enabling the use of the technologies routinely employed and optimized for each type of structure.
FIG. 3 is a schematic top view of an integrated optical circuit having an active area ZA and a passive area ZP on which are disposed masks 10 and 12 of two types respectively used to produce an active waveguide strip and to produce two passive waveguide strips.
The realignment method consists in protecting a portion of the active area ZA and all of the passive area ZP by means of a mask 10 and then etching the optical guide of the active structure using the appropriate and optimized etching technology routinely employed. In a second step a portion of the passive area ZP is protected by means of one or more other masks 12 and all of the active area ZA is protected with resin, for example, after which the optical guide of the passive structure is etched using the appropriate and optimized etching technology routinely employed.
The example shown schematically in FIGS. 4A through 4E explains how the various waveguides are defined. In a first step an active structure and two passive structures on respective opposite sites of the active structure are integrated onto a substrate 1 using the butt coupling method as previously explained with reference to FIGS. 1A through 1D. The schematics of FIGS. 4A through 4C correspond to a view of the integrated circuit in cross-section through the active structure. The schematics of FIGS. 4D and 4E correspond to a view of the integrated circuit in cross-section through the passive structure.
The realignment method consists in, in a first step, protecting all of the passive area and a portion of the active structure by means of a mask 10 and then etching the layers 6, 7, 8 on respective opposite sides of the mask 10. The active layer strip obtained in this way is then buried in a cladding layer 13, as shown in FIG. 4B.
Proton implantation in the cladding layer 13 then increases the resistivity of the material and consequently improves the confinement of the current in the active layer. An electrode is formed by depositing metallization 11 on top of the active layer ribbon 7 to enable current to be injected into the active structure.
In a second step a mask 12 is placed on top of the passive structure in order to etch the passive guides (FIG. 4D).
Resin is first applied to the combination of active and passive structures in order to protect the active structure and to enable the masks 12 to be placed on the passive structure. The masks 12 comprise a quartz plate, for example, on which are drawn imprints corresponding to the passive guides to be obtained. The masks 12 are placed on the integrated circuit and their position is then adjusted to align the drawn imprints with the waveguide of the active component. This adjustment is imprecise because it is done by eye. When the masks 12 have been adjusted the resin is exposed and the imprint of the guides is transferred to the resin. The passive guide strips are then etched in accordance with the shape of the imprint after which the resin is removed using a solvent. Unlike the active layer strip 7, the passive structure waveguides 3 are not buried.
The realignment procedure just described is conventional and well known to the skilled person.
The realignment method is highly advantageous as it is consistent with the standard processes that have been optimized and adapted to the fabrication of each type of structure. However, it introduces great difficulties concerning the alignment of the various waveguides. It is very difficult to position correctly the masks that define the various guides since this positioning can only be done via a microscope, that is to say by eye. Aligning two waveguides with different structures to an error of less than 1 .mu.m (10.sup.-6 m) is very difficult.
FIG. 5 is a schematic top view of two guides with different structures aligned by the realignment method. The guide 3 is a passive structure guide, for example, and the guide 7 is an active structure guide. The two waveguides 7 and 3 are slightly misaligned but sufficiently so to generate high optical coupling losses due to poor overlapping of the optical modes M3, M7 of the waves issuing from the guides.
The present invention solves this second problem and proposes a process for fabricating integrated optical circuits in which optical coupling losses are very significantly reduced.